Transformation produces many changes in cell physiology. To study these changes we developed a new ultrastructural immunocytochemical localization technique which allows electron microscopic antibody localization to intracellular protein in the presence of well preserved morphology in cultured cells (EGS procedure). Using this technique, we have localized the contractile protein actin, myosin and filamin, and the structural proteins of 100 A filaments, microtubules (tubulin), and the bristle coat of coated endocytic vesicles. We have also localized the src gene product polypeptide p60src of SR-avian sarcoma virus transformed cells, and found it associated with the inner aspect of the plasma membranes, concentrated at surface ruffles and gap junctions. Similar experiments with p21src of Harvey-murine sarcoma virus transformed cells showed that this src protein is also located at the inner surface of the plasma membrane, but is not concentrated at junctional regions.